@michael-andreuzza Maybe I don’t understand what you’re trying to do, but it seems to me that you have things exactly backwards.
If you want /about.html to display as /about, then you don’t want redirects (which don’t do this), you want Pretty URLs in the Asset Optimization section of the Build & Deploy section in your Netlify dashboard.
It sounds as though what you should do is have – for example – /sponsors/index.html instead of /sponsors.html. When visitors hit this page, it would display in the URL field as /sponsors.